This invention relates to casting toxic waste for burial or other suitable isolation. The cost of burying or isolating toxic wastes is increasing at an alarming rate due in part to stringent government regulations and sound environmental policy which dictate that toxic wastes cannot be simply dumped in a standard landfill.
Techniques for separation and isolation of mixed waste, particularly low-level radioactive wastes (LLRW) mixed with other hazardous constituents, are discussed in detail in co-pending U.S. patent application Ser. No. 07/160,814 filed Feb. 26, 1988 now U.S. Pat. No. 4,897,221, by Frank Manchak, Jr.
Some particularly hazardous toxic wastes, such as those wastes which are radioactive, must be stored in environmentally sound containers to effectively isolate the waste from the environment. These containers are often then stored in remote burial sites, suitable repositories or vaults. The use of standard metal drums or containers as containers for toxic waste is well known in the prior art. These drums are inappropriate for use with certain mixed wastes containing radioactive materials, and chemicals listed in 40 CFR Part 261, such as corrosives.
Since low level radioactive waste (LLW) and transuranic waste (TRU) must be isolated for up to hundreds of years, the containers holding these wastes must last as long regardless of whether they are subjected to corrosive action from the contents within the containers or from the exterior environment such as salt water or other chemical attack which is frequently present in underground storage vaults.
Radioactive waste must be isolated with adequate shielding to protect persons handling the waste and the environment. The type of shielding required depends on the type of radiation emitted by the waste. The following are encountered:
Fast Neutrons:
Fast neutrons are those neutrons with energies roughly above 100,000 eV (electron volts). Hydrogen nuclei are an effective shielding material for fast neutrons.
Water is a good source of hydrogen, but it is not suitable for use as a long term shield in its free liquid form because of the danger of leaks. Concrete is also effective as a shield for fast neutrons because the bound water in concrete is a source of hydrogen nuclei and functions as a shielding agent.
Another suitable shielding material is polyethylene which contains more hydrogen atoms per cubic centimeter than any other substance.
Thermal Neutrons:
Thermal neutrons are those neutrons with energies roughly below 0.025 eV. The most effective way to stop thermal neutrons is to shield with B.sup.10 (Boron.sup.10). B.sup.10 may be mixed in its pure form directly with the waste, or it may be added in its powdered form to concrete to be used as a containment vessel for radioactive wastes. B.sup.10 may further be suspended in polyethylene to form a combined shield for fast and thermal neutrons.
Gamma Rays:
Gamma rays are quanta of electromagnetic wave energy having wavelengths from 0.005 to 1.40 Angstroms. Gamma rays are best shielded by elements with high atomic numbers.
Alpha and Beta Radiation:
An alpha particle is identical with the nucleus of the helium atom and consists therefore of two protons plus two neutrons bound together. A beta particle is identical with an ordinary electron. Both alpha and beta particles are strongly ionizing when moving and so lose energy rapidly in traversing through matter. Most alpha particles will traverse only a few centimeters of air before coming to rest. While more shielding is generally necessary to stop beta particles than is required to stop alpha particles, most standard containers manufactured to isolate waste would provide sufficient shielding for both alpha and beta radiation.
Much of the high cost of burying or otherwise isolating toxic waste comes not directly from the mass of the waste, but rather from its volume. Space in suitable repositories is limited, so waste and shielding material must be packed as tightly as possible. There are various techniques currently used to compact toxic waste for isolation. Screw compactors are one of the most common waste volume reduction devices, and at the federal facility at Oakridge Tenn., a 100 ton compactor is used to crush or compact waste filled containers to reduce the volume thereof. The crushed waste containers must then be isolated in a suitable repository. The volume reduction capabilities of different compaction methods varies, but none of them is able to approach compaction to the maximum possible density. Further reduction in the volume of the repository or storage space required can be obtained if the wastes are compacted into non-cylindrical containers such as hexagonal or octagonal shapes which can be placed or stacked adjacent each other in honeycomb fashion to eliminate useless dead space which occurs between adjacently placed cylindrical containers.
Centrifugal castings are very dense, and heat produced in the casting by chemical reactions of toxic organic or inorganic wastes or by radioactive decay causes various problems. Heat is often produced by exothermic reactions in waste and the rate at which heat is produced in a mass of radioactive waste is generally related to the density of the wastes contained in a container thereof.
In a densely cast mixture of toxic waste, a symmetrical casting is ordinarily hottest in its center, and its temperature decreases towards its outer periphery. Without the provision of means for removing heat from the castings, extreme temperature gradients existing between the center of the casting, and its outer periphery are likely to cause the casting to crack due to thermal stresses. Also, shielding materials such as polyethylene will chemically degrade and not perform the shielding function if the temperature of the casting rises above about 400.degree. F.
Most underground repositories, or vaults for storing radioactive wastes have been placed in rock formations deep below the earth's surface. The heat which is produced by the reactive wastes stored in these vaults builds up to intolerable levels because it can not dissipate through the surrounding rock. If this heat is not removed, the temperature of the entire underground repository will rise beyond a level where human beings cannot work and containers of radioactive waste will melt or crack, thus releasing radioactivity. Also, any volatile components present in the radioactive waste will volatilize, and can easily carry radiation into the water table or atmosphere.